Method for controlling access to a shared wireless medium for several connections

ABSTRACT

The invention concerns a method for admission control of connections made up of one or several flows to a shared wireless medium. It applies a criteria for each new connection to determine whether resources can be provided for this connection. Said criteria includes the steps: calculating a number (R) of retransmissions of frames which are needed, depending on: 
         a target application PDU error rate (ε i ); a data link layer mean error rate (BER); and a maximum size (L) of the transmitted frames;    calculating the achievable quality of service parameters (T′ i ) based on the calculated number (R) of retransmissions with the target application PDU error rate (ε i ), and    determining if resources can be provided for the connection depending on the or each achievable quality of service parameter (T′ i ).

The present invention concerns an admission control method of connections made up of one or several flows to a shared wireless medium, each connection requiring a predetermined target application Protocol Data Unit error rate and at least an additional quality of service requirement, the transmission on the shared wireless medium being adapted for transmitting frames of a predetermined maximum size, with a data link layer mean error rate, the method applying a criteria for each new connection to determine whether resources can be provided for this connection. We assume an ARQ (Automatic Repeat reQuest) algorithm to be implemented at the data link layer level.

Such a method is used for Admission Control of connections with Quality of Service requirements.

By connection we mean the route, established between two different network devices, dedicated to the transmission of one or several data flows. When a connection transports several flows, each data link layer Protocol Data Unit (PDU) may include data issued from different flows.

To reach the Quality of Service requirements, the admission control method accepts to establish a new connection only if the load associated with this new connection is lower than the remaining admissible capacity on each shared link. Given C is the total capacity of a link and ρ is its admissible load, the admissible capacity of this link is ρ×C. Assume there are k connections that are already established on the link, and connection i uses a bandwidth equal to D′_(i), then the remaining admissible capacity equals ${\rho \times C} - {\sum\limits_{i = 1}^{k}{D_{i}^{\prime}.}}$

Each connection may have 0, 1 or several quality of service requirements which are necessary for satisfying each user after reception of his flow.

For example, if an application generates a flow with a minimum throughput requirement, the data link layer shall provide a bandwidth at least equal to this throughput.

In addition, some frames can be erroneous. These frames have to be retransmitted by the ARQ algorithm in order to reach a target application PDU error rate.

In the known Admission Control Methods, no means is provided in order to be sure that the shared link capacity is sufficient to enable all the retransmissions necessary for each connection sharing the link to reach its target application PDU error rate.

The aim of the invention is to provide a method for improving the management of the resources available on a link.

Accordingly, the subject of the invention is a method for admission control of connections on a shared wireless medium, characterized in that said criteria includes the steps:

calculating a number of retransmissions of frames which are needed in order to reach the target application error rate and the or each quality of service requirement, the number of retransmissions depending on:

the target application error rate;

the data link layer mean error rate; and

the maximum size of the transmitted frames;

calculating the achievable quality of service parameters based on the calculated number of retransmissions with the target application error rate, and

determining if resources can be provided for the connection depending on the or each achievable quality of service parameter.

According to particular embodiments, the method comprises the features of one or more sub-claims.

The invention will be better understood on reading the description which follows, given merely by way of example and while referring to the drawings in which:

FIG. 1 illustrates a network which aggregates several application PDU into a long frame;

FIG. 2 is a flowchart of an admission control method according to a first implementation of the invention;

FIG. 3 is a flowchart of an admission control method according to a second implementation of the invention;

FIG. 4 illustrates a network which segments each application PDU into several short frames;

FIG. 5 is a flowchart of an admission control method according to a third implementation of the invention, and

FIG. 6 is a flowchart of an admission control method according to a fourth implementation of the invention.

FIG. 1 shows a wireless telecommunication network 10 comprising a sending entity 12 adapted to communicate over an air interface with a receiving entity 14. This network can for example be an IEEE 802.11 WLAN.

The sending entity 12 comprises an emitter 16 which is adapted for sending long frames (LF) of maximum size L bytes to the receiving entity 14. The data link layer is assumed to have a mean bit error rate (BER).

The sending entity 12 includes several types of services 18 a, 18 b, 18 c and 18 d linked to the emitter 16. Each type of service may provide one or several flows to the emitter 16 in order to be sent to the receiving entity 14. The type of service 18 d is dedicated to best-effort services. In this case, no guarantee is provided to the users, and the present invention does not apply.

Each flow which type of service is either 18 a, 18 b or 18 c has to be received by the receiving entity 14 with a maximum application PDU error rate ε_(i) in order to satisfy the user.

In order to reach the maximum application PDU error rate ε_(i), the data link layer is adapted for re-transmitting the erroneous frames, when they are detected by the receiving entity. When an erroneous frame is received, the full frame is retransmitted according to the data link layer protocol.

In addition, one or several quality of service requirements has to be reached for each connection.

For example, for a voice service, the end-to-end transmission delay has to be lower than a maximum tolerated delay D_(i).

For a videoconference service, the bandwidth allocated to the connection has to guarantee a minimum throughput T_(i) at the application level.

Depending on the services, throughput or delay requirements may have to be reached in addition to the target application PDU error rate ε_(i).

The quality of service requirements D_(i), T_(i) and the target application PDU error rate ε_(i) have to be satisfied at the output of the receiving entity.

To ensure that the maximum application PDU error rate ε_(i) and a quality of service requirement D_(i) and/or T_(i) are reached, the emitter 16 comprises an admission control unit 20 which is in charge of implementing an admission control method as disclosed below.

It is assumed that an application outputs PDU TS of maximum size T bytes.

In case of long frames, the application PDUs are aggregated by the emitter 16 in order to make new frames LF of maximum size L bytes as shown on FIG. 1. The maximum size L of each new aggregated frame LF is higher than the maximum size T of each output application PDU TS plus all intermediate layers headers.

A first implementation of the admission control method is disclosed on FIG. 2. The method enables a minimum throughput T_(i) to be reached together with a maximum transmission application PDU error rate ε_(i).

At first stage 100, the number R of retransmissions needed for reaching the target application PDU error rate ε_(i) is calculated as follows: ${R = \left\lceil {\frac{\ln\left( ɛ_{i} \right)}{\ln\left( {1 - \left( {1 - {\varphi({BER})}} \right)^{8L}} \right)} - 1} \right\rceil},$ where φ is a function. It can for example | be φ(x)=x, ∀x or φ(x)=αx, ∀x, with α∈R.

Let κ_(i) be the traffic induced while crossing intermediate layers.

At step 102, the bandwidth T′_(i) to reserve at the data link layer level in order to reach the quality of service requirement(s) is determined based on the application throughput T_(i), the number of retransmissions R and the parameter κ_(i).

The bandwidth T′_(i) is defined as: T′ _(i)=(1+R)×(T _(i)+κ_(i)).

In case application PDU are emitted according to a periodic profile (P_(i) seconds between two consecutive application PDUs), and H_(i) represent the size of intermediate layers headers in bits, κ_(i) can be computed as $\kappa_{i} = {\frac{H_{i}}{P_{i}}.}$

At step 104, it is checked whether the remaining link capacity is sufficient to allocate bandwidth T′_(k+1) to the new connection. For example, if C is the capacity of the link from the emitter 16 to the receiver 14 and ρ is the admissible load on this link, it is determined, at step 104, if the link still has enough admissible capacity left to reserve bandwidth T′_(k+1) to the new connection. Assuming that each connection i needs a bandwidth T′_(i), the remaining admissible capacity equals ${\rho \times C} - {\sum\limits_{i = 1}^{k}{T_{i}^{\prime}.}}$ Thus it is checked if $T_{k + 1}^{\prime} < {{\rho \times C} - {\sum\limits_{i = 1}^{k}{T_{i}^{\prime}.}}}$

If the capacity is sufficient, the new connection is established at step 106, otherwise the connection is refused at step 108.

FIG. 3 shows the algorithm of the method enabling an end-to-end transmission delay requirement D_(i) to be reached together with a maximum transmission application PDU error rate ε_(i).

As previously disclosed, the number R of needed retransmissions is calculated at step 200 as a function of ε_(i), BER and L.

At step 202, the transmission duration induced by the retransmission is estimated as: D′_(i)=(R+1)×RTT where RTT is the Round Trip Time, that is to say the end to end time transmission delay from emitter to receiver and for the reverse path, including processing and waiting times, for each LF frame.

The admission control method determines at step 204 if the new calculated end-to-end transmission delay D′_(i) is lower than the required maximum end-to-end transmission delay D_(i). If D′_(i)<D_(i), the connection is established at step 206, otherwise, the connection is refused at step 208.

Two different quality of service requirements are disclosed in the algorithms of FIGS. 2 and 3, each requirement being reached separately. In fact, both requirements may be needed and in this case the admission control method allows a connection to be established only if both requirements are met.

FIG. 4 shows another wireless telecommunication network which segments application PDU. It can be for example an HiperLAN2 WLAN.

On FIG. 4, the same reference numerals refer to the same units as on FIG. 1.

In case of segmentation, the data link layer PDU are short frames denoted by SF. Their maximum size in bytes is denoted by S and is lower than the maximum size T of the application PDU denoted TS. Consequently, each application PDU TS plus intermediate layers headers are segmented by the emitter 16 in n short frames SF.

In case of segmentation, the number R of retransmissions is shared out between the n short frames SF that make up the TS application PDU.

Define α(n, R) by ${{\alpha\left( {n,R} \right)} = {\sum\limits_{i = 1}^{\inf{({n,R})}}{C_{n}^{i}C_{R - 1}^{i - 1}}}},$ ∀n, R≧1, and LER by LER=1−(1−φ(BER))^(8.S), where φ is a function. It can for example by φ(x)=x, ∀x or φ(x)=αx, ∀x, with α∈R.

Let E_(r) be the event “the application PDU TS is erroneous after r SF retransmissions”.

The probability IP(E_(r)) is computed by induction as follows: ${{IP}\left( E_{r} \right)} = \left\{ \begin{matrix} {1 - \left( {1 - {LER}} \right)^{n}} & {{{if}\quad r} = 0} \\ {{{IP}\left( E_{r - 1} \right)} - {{\alpha\left( {n,r} \right)}{LER}^{r} \times {{IP}\left( E_{0} \right)}}} & {\forall{r \geq 1.}} \end{matrix} \right.$

To compute the number of retransmissions needed to reach an application PDU error rate target value, IP(E_(r)) is computed by induction until R=inf {r∈IN: IP (E_(r))≦ε_(i)} is reached. The number of retransmitted SF is given by R, whereas the total number of transmitted SF equals R+n.

An implementation of the admission control is disclosed in FIG. 5. The method enables a maximum throughput T_(i) to be reached together with a maximum transmission application PDU error rate ε_(i).

At step 300, the number R of retransmissions needed is calculated as explained above as a function of ε_(i), BER, n and S.

In order to reach the required throughput T_(i), the bandwidth to reserve T′_(i) is calculated in that case at step 302 as ${T_{i}^{\prime} = {\left( {1 + \frac{R}{n}} \right) \times \left( {T_{i} + \kappa_{i}} \right)}},$ where κ_(i) is the traffic induced while crossing intermediate layers. In case application PDU are emitted according to a periodic profile (P_(i) seconds between two consecutive application PDUs), and H_(i) represent the size of intermediate layers headers in bits, this parameter can be computed as $\kappa_{i} = {\frac{H_{i}}{P_{i}}.}$

A checking step 304 and a connexion step 306 or a connexion refusal step 308 corresponding to steps 104, 106 and 108 of FIG. 2 are then implemented.

A rough upper bound of delay may be calculated as in the previous case described in FIG. 3 where D′_(i) is computed as D′_(i)=(1+R) RTT.

FIG. 6 shows an alternative algorithm of the method enabling an end-to-end transmission delay requirement D_(i) to be reached together with a maximum transmission application PDU error rate ε_(i). In that case, D′_(i) is computed more accurately by using probability theory.

A stage 400, the number R of needed retransmissions is calculated as a function of ε_(i), BER, n and S.

At stage 402, D′_(i) is calculated as a probability distribution function.

In this case, we have: $\begin{matrix} {{{IP}\left( {D_{i}^{\prime} \leq d} \right)} = {\sum\limits_{i = 0}^{\inf{({n,R})}}{\sum\limits_{k = i}^{R}{\sum\limits_{{r_{1} + \ldots + r_{i}} = k}{1{I_{\lbrack{{\max{\{{r_{1},\ldots\quad,r_{i}}\}} \times {RTT}},{+ {\infty\lbrack}}}}(d)} \times}}}}} \\ {C_{n}^{i}{{LER}^{k}\left( {1 - {LER}} \right)}^{n}} \end{matrix}$ where 1I_(A)(x)=1 if and only if x lies in the subset A.

At step 404, a maximum delay D_(δ) is calculated according to an allowed margin of error denoted δ.

D_(δ) is chosen such that IP(D′ _(i) ≧D _(δ))≦δ.

At step 406, a test is carried out so that the new connection is accepted at step 408 provided its required delay D_(i) is lower than or equal to D_(δ). Otherwise, the connection is refused at step 410. 

1. Method for admission control of connections made up of one or several flows to a shared wireless medium, each connection requiring a predetermined target application Protocol Data Unit (PDU) error rate (ε_(i)) and at least an additional quality of service requirement (D_(i); T_(i)), the transmission on the shared wireless medium being adapted for transmitting frames (LF; SF) of a predetermined maximum size (L; S), with a data link layer mean error rate (BER), the method applying a criteria for each new connection to determine whether resources can be provided for this connection, characterized in that: said criteria includes the steps: calculating a number (R) of retransmissions of frames (LF; SF) which are needed in order to reach the target application PDU error rate (ε_(i)) and the or each quality of service requirement (D_(i); T_(i)), the number (R) of retransmissions depending on: the target application PDU error rate (ε_(i)); the data link layer mean error rate (BER) ; and the maximum size (L; S) of the transmitted frames (LF; SF); calculating the achievable quality of service parameters (D′_(i); T′_(i)) based on the calculated number (R) of retransmissions with the target application PDU error rate (ε_(i)), and determining if resources can be provided for the connection depending on the or each achievable quality of service parameter (D′_(i); T′_(i)).
 2. Method according to claim 1, characterized in that a quality of service requirement is the maximum end-to-end transmission delay (D_(i)) required by a connection.
 3. Method according to claim 1, characterized in that a quality of service requirement is the minimum throughput (T_(i)) required by a connection.
 4. Method according to any one of the preceding claims, characterized in that several application PDU are aggregated into long frames (LF), the maximum size (L) of the long frames (LF) transmitted over the shared wireless medium being higher than the maximum size (T) of the application PDU plus all intermediate layers headers and in that the number (R) of needed retransmissions is: $R = \left\lceil {\frac{\ln\left( ɛ_{i} \right)}{\ln\left( {1 - \left( {1 - {\varphi({BER})}} \right)^{8L}} \right)} - 1} \right\rceil$ where: BER is the mean bit error rate on the data link layer; L is the maximum size of transmitted data link layer PDU (LF) in bytes; ε_(i) is the target application PDU error rate; and φ is a function.
 5. Method according to claim 2 and 4, characterized in that the achievable maximum end-to-end transmission delay D′_(i) is: D′_(i)=(1+R) RTT, where RTT is the round trip time for each long frame (LF).
 6. Method according to claims 3 and 4, characterized in that the bandwidth to reserve T′_(i) is T′_(i)=(1+R)×(T_(i)+κ_(i)), where D_(i) is the application throughput and κ_(i) is the additional bandwidth induced by intermediate layers.
 7. Method according to any one of claims 1-3, characterized in that each application PDU is segmented into several short frames (SF), the maximum size (S) of the short frames (SF) transmitted over the shared wireless medium being lower than the maximum size (T) of the application PDU (TS) plus all intermediate layers headers leading to each application PDU (TS) to be segmented in n short frames (SF) and in that the number (R) of needed retransmissions is calculated by induction until a threshold (ε_(i)) is reached.
 8. Method according to claim 7, characterized in that the number R of needed retransmissions is defined by computing by induction on r the probability IP(E_(r)) of the event “the transmitted application PDU TS is erroneous after r SF retransmission” until we reach R=inf {rεIN: IP (E_(r))≦ε_(i)}.
 9. Method according to claim 8, characterized in that: LER=1−(1−φ(BER))^(8.S) where BER is the mean bit error rate on the data link layer, φ is a function, and ${{IP}\left( E_{r} \right)} = \left\{ \begin{matrix} {1 - \left( {1 - {LER}} \right)^{n}} & {{{if}\quad r} = 0} \\ {{{IP}\left( E_{r - 1} \right)} - {{\alpha\left( {n,r} \right)}{LER}^{r} \times {{IP}\left( E_{0} \right)}}} & {\forall{r \geq 1.}} \end{matrix} \right.$
 10. Method according to claims 1, 3, 7, 8 and 9, characterized in that the bandwidth to reserve ${{T_{i}^{\prime}\quad{is}\quad T_{i}^{\prime}} = {\left( {1 + \frac{R}{n}} \right) \times \left( {T_{i} + \kappa_{i}} \right)}},$ where T_(i) is the application throughput and κ_(i) is the additional bandwidth induced by intermediate layers.
 11. Method according to claims 1, 2, 7, 8 and 9, characterized in that the achievable maximum end-to-end transmission delay D′_(i) is: D′_(i)=(1+R) RTT, where RTT is the round trip time for each short frame (SF).
 12. Method according to claims 1, 2, 7, 8 and 9, characterized in that the probability distribution function of the end-to-end transmission delay (D′_(i)) is: ${{{IP}\left( {D_{i}^{\prime} \leq d} \right)} = {\sum\limits_{i = 0}^{\inf{({n,R})}}{\sum\limits_{k = i}^{R}{\sum\limits_{{r_{1} + \ldots + r_{i}} = k}{1{I_{\lbrack{{\max{\{{r_{1},\ldots,r_{i}}\rbrack} \times {RTT}},{+ {\infty\lbrack}}}}(d)} \times \quad C_{n}^{i}{{LER}^{k}\left( {1 - {LER}} \right)}^{n}}}}}},$ where R is previously fixed and 1I_(A)(x)=1 if and only if x∈A.
 13. Method according to claim 12, characterized in that the allowed margin of error is δ, and the achievable end-to-end transmission delay (D′₆₇ ) is determined by IP(D′_(i)>D′_(δ))≦δ.
 14. Computer program product for an admission control unit, comprising a set of instructions which, when loaded into said admission control unit, causes the admission control unit to carry out the method according to any one of the preceding claims.
 15. Admission control unit for controlling the admission of connections to a shared wireless medium, each connection being made up of one or several flows requiring a predetermined target application Protocol Data Unit (PDU) error rate (ε_(i)) and at least an additional quality of service requirement (D_(i); T_(i)), the data link layer being adapted for transmitting frames (LF; SF) of a predetermined maximum size (L; S), with an error rate (BER), the unit comprising means for applying a criteria for each new connection to determine whether resources can be provided for the establishment of the new connection, characterised in that: said means for applying a criteria includes: means for calculating a number (R) of retransmissions of frames (LF: SF) which are needed in order to reach the target application PDU error rate (ε_(i)) and the or each quality of service requirement (D_(i); T_(i)), the number (R) of retransmissions depending on: the target application PDU error rate (ε₁); the data link layer mean error rate (BER); and the maximum size (L; S) of the frames (LF; SF), means for calculating the achievable quality of service parameter (D′₁; T′_(i)) based on the calculated number (R) of retransmissions with the target application PDU error rate (ε_(i)), and means determining if resources can be provided to the connection depending on the or each achievable quality of service parameter (D′_(i); T′_(i)). 